A storm surge is an abnormal rise in coastal sea level above the astronomical tide that is produced primarily by a storm. It occurs when atmospheric pressure and wind associated with a storm cause seawater to accumulate towards a coastline. The term surge specifically refers to the storm driven increase in water level, rather than the ordinary astronomical rise and fall caused by the gravitational effects of the Moon and Sun.
Storm surge is particularly associated with tropical cyclones, hurricanes and typhoons, but strong extratropical cyclones can also generate substantial surges. The observed coastal water level during a storm may be described as the combination of astronomical tide and storm surge, with waves producing additional effects on top of that level. Because the surge can coincide with high tide, even a moderate atmospheric disturbance can produce dangerous coastal flooding if the timing is unfavourable. Storm surge is therefore a water level phenomenon generated by atmospheric forcing and modified by coastal geography and ocean depth.
Wind stress is usually the dominant physical mechanism producing storm surge. Strong winds blowing towards a coastline push water in the direction of the wind and create a local accumulation of seawater. The effect becomes especially important over shallow continental shelves because water cannot easily move downward and is therefore displaced horizontally towards the coast. The geometry of the coastline and sea floor can amplify this process, particularly in funnel shaped bays, estuaries and shallow coastal embayments.
Atmospheric pressure also contributes because lower pressure reduces the downward force exerted on the ocean surface, allowing the sea surface to rise slightly in response to the pressure anomaly. The pressure contribution is generally smaller than the wind contribution in many major storms, but it can become significant in intense systems. The final surge height results from the combined action of wind, pressure, storm structure, ocean depth, coastline geometry and the movement of the storm.
Storm surge should be distinguished carefully from storm tide and from wave runup. Storm surge is the abnormal water level caused by the storm itself after separating out the predictable astronomical tide. Storm tide is commonly used to describe the combined water level produced by astronomical tide and storm surge. Wave runup is the uprush of individual waves onto a beach, dune or structure and can raise the instantaneous water reach substantially above the underlying still water level.
Wave setup is a more sustained increase in mean water level near the shoreline caused by the breaking of waves. During a severe coastal storm, all of these processes can occur together. A coastal community may therefore experience flooding even when the surge alone does not appear exceptionally high because high tide, wave setup and wave runup add to the water level. Understanding these distinctions is essential when interpreting forecasts, tide gauges, flood maps and historical reports of storm flooding.
The shape and depth of the continental shelf have a major influence on storm surge. A broad shallow shelf can allow wind to push a large volume of water towards the coast while limiting the ability of that water to escape into deeper ocean. In contrast, a steep continental shelf may permit water to move more readily into deeper water, potentially reducing the local wind driven accumulation, although other factors remain important.
Bays and estuaries can amplify surge through geographical funneling, particularly when the coastline narrows inland. The orientation of the coast relative to the storm wind field is also crucial. A coastline facing the direction in which strong onshore winds push water may experience a much larger surge than a nearby coastline exposed to offshore or alongshore winds. Small differences in storm track can therefore produce very large differences in local surge heights, especially near complex coastlines.
Storm surge varies strongly with the structure and movement of the storm. A tropical cyclone contains a large wind field that may extend far beyond the region of maximum wind speed, so substantial surge can occur over a wide area. The storms forward speed also matters because a slowly moving storm can maintain strong winds over a particular coastal region for longer, allowing more water to accumulate, although the relationship is not simple.
The direction in which the storm moves relative to the coastline can determine which side experiences the strongest onshore wind. In the Northern Hemisphere, the right side of a moving tropical cyclone is often associated with stronger onshore winds in particular coastal configurations because the storms rotational circulation and forward motion reinforce one another, although local geometry determines the actual outcome. Forecasting surge therefore requires detailed knowledge of storm intensity, size, track, speed, wind field and coastal bathymetry.
Storm surge can produce some of the most damaging effects of coastal storms because water can cover large low lying areas and exert strong forces on buildings, roads, bridges and coastal defences. Surge can also transport debris inland, erode beaches and dunes, inundate freshwater sources and interfere with drainage. The danger increases when surge arrives together with large waves because wave action can repeatedly strike and overtop structures while the elevated water level allows waves to reach farther inland.
Surge can also combine with river flooding, particularly in estuaries where high coastal water levels can slow or temporarily reverse the discharge of rivers. This interaction is called compound flooding. A location may therefore experience flooding from several sources simultaneously, and the resulting water level cannot always be estimated by considering each hazard independently. Modern coastal forecasting increasingly uses coupled atmospheric, oceanographic and hydrological models to represent these interactions.
Historical storm surges have influenced the design of coastal settlements and engineering systems in many parts of the world. Tide gauges provide long term records of coastal water levels, while modern satellite observations, numerical ocean models and meteorological forecasts provide additional information about surge conditions. Coastal engineers use extreme water level statistics to estimate design conditions for sea walls, flood barriers, ports, drainage systems and other infrastructure.
Return periods are commonly used to express the rarity of extreme water levels, although a return period is a statistical property rather than a prediction that an event will occur on a fixed schedule. Climate change adds further complexity because rising mean sea level raises the baseline upon which future surges occur. Even if the characteristics of individual storms remain uncertain, higher background sea levels can increase the probability that existing surge events produce damaging inundation.
Storm surge is therefore best understood as a dynamic interaction between atmosphere, ocean and coastal geography. It is not simply a large wave and it is not synonymous with high tide. The central mechanism is the storm induced redistribution of seawater, primarily through wind stress and secondarily through atmospheric pressure effects, with coastal bathymetry and geometry determining how much water accumulates locally. Its practical significance depends on the total water level reached when surge combines with tide and wave processes.
This is why surge forecasts normally provide information relative to a predicted tide and why emergency planning focuses on inundation rather than surge height alone. A physically similar storm can produce very different coastal impacts depending on its track, timing, intensity, size, coastline orientation, shelf depth and local elevation.
